Hot Swap Controller Application Introduction

"Hot plugging" refers to inserting or removing a card from a powered host (backplane, server, etc.). It is mainly used in base stations, redundant arrays of disks (RAID),

Remote access server, network router, network switch and ISDN system. When the board is inserted into the host, the host is already in a steady-state working state, all capacitors are fully charged, and the circuit board to be inserted is not charged, and the capacitors on the board have no charge. Therefore, when the board contacts the back panel of the host, the charging of the capacitors on the board will draw a large transient current from the host power supply; similarly, when the charged board is pulled out of the host, the discharge of the bypass capacitor on the board forms a low-resistance path between the board and the charged back panel, which will also generate a large transient current. Large currents can cause damage to components or devices such as connectors, circuit elements, and metal connections (traces) on circuit boards, and may also cause a transient drop in the backplane power supply, resulting in a system reset. At present, there are many new hot-swap protection devices for the above applications. Maxim's MAX4273 series has dual-speed/dual-level detection functions, which can provide an effective control and protection solution for hot-swap applications.

1 Internal structure and function of MAX4273

The internal circuit of MAX4273 is shown in Figure 1, which includes: charge pump, low-speed comparator, high-speed comparator, undervoltage/overvoltage detection circuit, logic controller, etc. The charge pump is used to provide driving voltage for the gate of external N-channel MOSFET. The low-speed comparator and high-speed comparator are used to provide dual-speed/dual-level overload or fault current detection. The response time of the low-speed comparator is set by an external capacitor, and the setting range can be from 20μs to several seconds. The voltage detection threshold is fixed at 50mV. For transient overload currents with lower amplitudes, the low-speed comparator has no response and is not affected by small fluctuations in the power supply voltage and noise. When the device detects that the overload current time exceeds the set response time, it is considered that the system has failed.

At this time, the gate of the MOSFET begins to discharge slowly, and eventually disconnects the MOSFET. The discharge rate is determined by the gate capacitance of the N-channel MOSFET and the external capacitor. The response time of the high-speed comparator is fixed at 350ns. The voltage detection threshold can be set by the external circuit RTH. The setting range is 50mV to 750mV. Once a large-amplitude fault current is detected, the high-speed comparator will directly and quickly disconnect the MOSFET. Generally, the voltage detection threshold of the high-speed comparator should be higher than the voltage detection threshold of the low-speed comparator to handle sudden faults. The undervoltage detection circuit is used to detect whether the input voltage (VIN) is higher than the undervoltage lockout output (UVLO) threshold (minimum value 2.25V). If VIN>VUVLO and the delay time does not reach 150ms, the MAX4273 will limit the conduction of the MOSFET to avoid insufficient drive of the external MOSFET gate. The 150ms delay time is used to ensure that VIN can reach a stable state after the board is fully inserted into the back panel of the host. Once VIN is lower than the UVLO threshold, the chip will be reset and initialize a startup sequence. The overvoltage detection circuit ensures that when a system fault is detected, the chip will not restart until the load voltage is lower than 0.1V after the MOSFET gate voltage is fully discharged (voltage lower than 0.1V). The rated voltage VGS between the gate-source voltage of the new MOSFET is added as a limit to protect the MOSFET from damage.

2 MAX4273 Control Timing

The capacitor CTON in Figure 1 is used to set the startup cycle. When VIN>VUVLO and reaches 150ms, VON>0.6V, and the chip cannot retry, the controller is turned on. During the startup cycle, the low-speed comparator is disabled, and the MOSFET gate drive current is limited to less than 100μA and decreases as the gate voltage increases.

MAX4273 can provide load current limiting in two ways: the first is to control the gate voltage of MOSFET to make the load current rise slowly; the second is to limit the load current by adjusting the external current limiting resistor. During the startup process, the high-speed comparator is part of the current regulation loop. When the load current is higher than the detection threshold, the gate voltage of MOSFET will discharge at a current of 70μA. When the load current is lower than the allowed limit, the charge pump will turn on again. Therefore, the load current will fluctuate around the threshold value set by the fast comparator. The rise or fall time is determined by the high-speed comparator and the charge pump transmission delay time. At this time, the load current will show a 20% ripple. Increasing the capacitance between the MOSFET gate and GND can reduce the ripple. After the startup cycle is over, the controller enters the normal operation mode, and the status output pin STAT of MAX4273 outputs a high level. At this time, if the low-speed comparator or the high-speed comparator detects a fault current, STAT will become a low level, and the output voltage will be discharged through the internal 1kΩ resistor between LLMON and GND. Its fault response timing is shown in Figure 2.

3 Hot-swap protection circuit design

The hot-swap controller can be placed on the board as shown in Figure 3 or on the backplane as shown in Figure 4. When placed on the backplane, it allows boards with different input capacitance (without hot-swap protection) to be plugged in and out of the same slot. The circuit in Figure 3 can use the ON comparator inside the MAX4273 to monitor the temperature of external components (such as MOSFET). When the temperature exceeds the preset threshold, the ON comparator disconnects the MOSFET through the logic controller. The reset comparator monitors the output voltage to provide reset control for the microprocessor. When designing specifically, attention should be paid to the selection of external components and the setting of relevant parameters.

3.1 Reasonable selection of external components

N-channel MOSFETs should be selected with low on-resistance (RDS(ON)) to ensure a low voltage difference between the drain and source at full load, thereby reducing the power loss of the MOSFET. If RDS(ON) is large, the output voltage will fluctuate with the change of the board load. Table 1 lists several recommended MOSFET models for design reference.

Table 1 Recommended MOSFET models:

When selecting the current limiting resistor (RSENSE), it should be ensured that the voltage drop caused by the maximum operating current allowed on the current limiting resistor is higher than the overload voltage threshold (50mV) of the low-speed comparator. Usually the overload current is set to 1.2 to 1.5 times the maximum operating current. The threshold voltage of the high-speed comparator should be fixed at 220mV or adjusted between 50mV and 4750mV. The fault detection current is generally set to 4 times the overload current threshold. Table 2 lists several RSENSE values ​​corresponding to the current limiting level:

Table 2 Current limiting level and RSENSE:

a. Fast comparator threshold setting

The fast comparator fault detection threshold is determined by RTH, and the threshold adjustable range is 50mV~750mV, and the corresponding resistance value range is 5k~75kΩ, RTH (kΩ) = VTH.FC (mV) / 10. Note that when 200Ω

b. Start time setting

CTON in Figure 3 determines the maximum startup time allowed by MAX4273. The default value (CTON pin floating) is μs, and the startup time is tON (ms) = 0.31xCTON (nF). When selecting CTON, tON should be large enough to ensure that the MOSFET has sufficient gate drive capability within the startup time and the load capacitor is fully charged.